A fundamental feature in cellular radio communication systems is handover (HO). Handover is a main function that is used to support mobility in the network. When a mobile terminal or user equipment (UE) is moving in the cellular network, it has to change serving cell when the signal from a current cell is too weak to support the current radio link and/or when it is decided that another cell has better possibilities to support the radio communication for the user terminal.
LTE (Long Time Evolution) is a radio access technology standardized by the 3GPP (3d Generation Partnership Project). An LTE system uses orthogonal frequency division multiplex (OFDM) as a multiple access technique (called OFDMA) in the downlink from system nodes to user equipment. An LTE system typically has channel bandwidths ranging from about 1.4 MHZ to 20 MHz, and supports throughputs of more than 100 megabits per second on the largest-bandwidth channels. One type of physical channel defined for the LTE downlink is the physical downlink shared channel (PDSCH), which conveys information from higher layers in the LTE protocol stack and to which one or more specific transport channels are mapped. Control information is conveyed by a physical uplink control channel (PUCCH) and by a physical downlink control channel (PDCCH).
The LTE radio access architecture is based around LTE radio base stations, referred to as eNodeB, which communicate with mobile terminals, also referred to as User Equipment (UE).
In LTE (as well as in WCDMA) the so-called Radio Resource Control (RRC) protocol performs a plurality of functions between the user equipment and the base station e.g. eNodeB. Amongst those functions can be mentioned broadcasting system information related to NAS (Non-Access Stratum) and AS (Access Stratum), establishment, maintenance, and release of RRC connection, establishment, configuration, maintenance, and release of signaling and data radio bearers. Of these functions, the management of the RRC connection is of most interest for the current disclosure.
In LTE a user equipment (UE) can be in two different states as illustrated in FIG. 1. One state, the RRC_CONNECTED is the state used when the user equipment is active and connected to a specific cell e.g. eNodeB within the network. One or several IP addresses have been assigned to the user equipment, as well as an identity of the user equipment, the Cell Radio Network Temporary Identifier (C-RNTI), used for signaling purposes between the user equipment and the network. Although expressed differently in the specification, the RRC_CONNECTED state can be said to include two sub-states, namely IN_SYNC and OUT_OF_SYNC, depending on whether the uplink is synchronized to the network or not. Since LTE uses an orthogonal FDMA/TDMA (Frequency Division Multiple Access/Time Division Multiple Access) based uplink, it is necessary to synchronize the uplink transmission from different mobile terminals or user equipment to ensure that they arrive at the receiver at the same time. In short, the receiver measures the arrival time of the transmissions from each actively transmitting mobile terminal and sends timing-correction commands in the downlink. As long as the uplink is synchronized, uplink transmission for user data and L1/L2 (Layer 1/Layer 2) control signaling is possible. In case no uplink transmission has taken place within a given time window, timing alignment is obviously not possible and the uplink is declared to be non-synchronized. In this case, the mobile terminal needs to perform a random-access procedure to restore uplink synchronization. The state RRC_IDLE is a so-called low activity state in which the UE sleeps, i.e. is inactive, most of the time in order to reduce battery consumption. Uplink synchronization is not maintained and hence the only uplink transmission activity that may take place is random access to move from RRC-IDLE to RRC_CONNECTED. In the downlink, the UE can wake up, i.e. become active, periodically in order monitor the Paging Channel (PCH) according to what is commonly referred to as Discontinuous Reception Cycle (DRX) in order to be paged for incoming calls, as will be described in more detail below. The mobile terminal maintains its IP address(es) and other internal information in order to rapidly move to RRC_CONNECTED when necessary.
The above mentioned discontinuous reception cycle (DRX) is a functionality that enables reduced power consumption in order for the UEs to reduce their power consumption to save UE battery time. The DRX mechanism allows the UE to sleep most of the time, with the UE receiver circuitry switched off, and only periodically wake up for a brief period to monitor the paging channel. In essence, the DRX comprises a periodic repetition of an “on duration” period followed by a possible period of inactivity or sleep. The “on duration” defines periods of mandatory activity. Preferably, the UE is configured with an on duration of 2 ms per 20 ms. During the active periods the UE receives assignments or grants for new data after which an inactivity timer is started and the UE is prepared to be scheduled continuously. Other active periods are when the UE is expecting a retransmission of a downlink HARQ (Hybrid Automatic Retransmission reQuest) transmission, or HARQ feedback for an uplink HARQ transmission, or after transmitting a scheduling request. A known DRX scheme includes two levels of inactivity, namely, a long DRX for power efficient operation during periods of low activity, and a short DRX for low latency during periods of more activity. For a UE in the RRC_IDLE state a DRX pattern aligned to the basic paging schedule is applied on a group basis for a set of UEs. The DRX pattern is aligned to the paging schedule in such a way that the UE has a possibility to read the paging messages while awake rather than while in the battery saving DRX sleep mode.
The two above described RRC states of a UE can be summarized according to the following:                RRC_CONNECTED: The UE RRC connection is maintained/controlled by the network. Handover procedure is used for mobility management, and the UE wakes up/sleeps according to the configured DRX parameters.        RRC_IDLE: The UE RRC connection is released, so no UE related information is stored in the eNodeB side. Once there is data to send/receive to/from the UE, the RRC connection has to be re-established. Cell re-selection is used for mobility management, and the UE wakes up/sleeps according to the paging interval.        
Based on the above it is evident that the different RRC states each cause different signaling load, i.e., more handover related signaling for connected UE, and more RRC re-establishment signaling for RRC idle UE. In addition, different RRC states mean different delay performance, since the reaction speed depends on the length of the DRX/Paging cycle.
The switching between the aforementioned two RRC states i.e. RRC_CONNECTED to RRC_IDLE, from idle to connected, and from connected to idle, is controlled by a network entity, typically the eNodeB.
A conventional known method of RRC state switching is to “release the UE to idle after N second UE inactivity”. That is, the value of N i.e. the inactivity timer, is taken as the threshold to decide UE activity/inactivity, which is also denoted as the RRC timer in this disclosure.
In a known method of managing RRC state switching, presented in the document R2-116036, Signaling considerations for background traffic, Qualcomm Incorporated, RAN2#76, Nov. 14-18, 2011, a RRC switching scheme is proposed, which essentially considers the low/high UE mobility difference in some degree. Briefly, the scheme is to “release the UE if 1) N seconds inactivity and 2) one or more HO observed in this N second”, which can be summarized according to the following:                UEs that have “HO in N inactive seconds” are treated as high mobility UEs, and will be released after N seconds.        UEs that have “No HO in N inactive seconds” are treated as low mobility UEs, and will be kept in connected mode.        
Hence, according to the prior art document R2-116036, Signaling considerations for background traffic, Qualcomm Incorporated, RAN2#76, Nov. 14-18, 2011, the UEs can be grouped according to their level of mobility into two levels, i.e., high/low mobility, and use two timers accordingly, i.e., N and an infinite timer.
One problem with the above described prior art switching method is that it is a very blunt instrument, which will cause a less than optimal performance at high mobility levels.
Based on the above, there is a need for an improved RRC state switching method, which enables an optimization of usage of radio resources, whilst at the same time maintaining a high level of service for connected user equipment.